hi I have been wondering about this question for some time, if a disease (such as sickle cell) is inherited as an X-linked recessive gene in humans and an unaffected woman with no history of the disease marries a man who has the disease, what propotion of their sons and their daughters are expected to be affected?..

zatom wrote:hi I have been wondering about this question for some time, if a disease (such as sickle cell) is inherited as an X-linked recessive gene in humans and an unaffected woman with no history of the disease marries a man who has the disease, what propotion of their sons and their daughters are expected to be affected?..

Is it still a simplictic 3:1 ratio?

Firstly sickel cell disease is not an X-linked recessive condition - it is an autosomal recessive condition... In the offspring there is a 25% chance of having disease, 50% of carrier, and 25% of not having disease... i think this is what you are referring to as 'the 3:1 ratio'...

An X-linked recessive condition would be something like Haemophilia...

Here is an example... The effected allele I've put in red

Male with haemophilia XY vs. Normal female XX

Offspring

50% chance of female carrier XX
50% chance of normal male XY

And so, none of the offspring would suffer from the condition in this case...

zatom wrote:hi I have been wondering about this question for some time, if a disease (such as sickle cell) is inherited as an X-linked recessive gene in humans and an unaffected woman with no history of the disease marries a man who has the disease, what propotion of their sons and their daughters are expected to be affected?..

Is it still a simplictic 3:1 ratio?

The example I gave you in your takehome quiz used G6PD deficiency, which is X-linked. Sickle cell anaemia isn't.

hi dude see your question is pretty valid and a good one that mite confuse many but it has a simple approach see what happens is that a male contributes only X chromosome to daughter and a Y chromosome to the son.so it means that the other half is the contribution of the mother.and if the male is suffering frm the disease so what will happen,he would have a progeny that has following

sons:- they will not carry the disease as its an X-linked disease and the father or the male doesnt contribute X chrmosome to his sons.

daughters:- in this case the daughters will be the carriers of the disease as they get an x- chromosome from the father amd the other half from their mother.

I ask essentially this question for one of my laboratory exercises, and the wrong answer of Revenged is often given - it's an easy mistake to make.

sportysunnyfunny has it right, but somehow the answer - no offspring affected, but all females carriers - doesn't seem right. I've seen many students cross out the right answer and substitute a wrong one.

The ratio you assumed is 3:1 but according to my mind it is not possible to get 3:1 ratio from a XX & XY cross.I think you are confuced with Aa & Aa cross.(dihybrid cross between heterozygous genotypes)

Saneth wrote:The ratio you assumed is 3:1 but according to my mind it is not possible to get 3:1 ratio from a XX & XY cross.I think you are confuced with Aa & Aa cross.(dihybrid cross between heterozygous genotypes)

ok, i see... i did wonder why people were getting confused... and yes, i was assuming that both the parents were carriers in my autosomal recessive ('3:1') example... i just didn't say it so, which is why i confused people!...